Any of you that are planning to do the 3.73 to 4.56 gear ratio swap and also have plans to do a dyno run:

It would be great if you could do a BEFORE and AFTER dyno run, with NO other changes made meantime.

The reason is that I think finally found a good explanation of why the horsepower reported by an inertia type dyno (i.e. any Dynojet dyno) DECREASES if:

1. The run is made in any gear lower than the 1 to 1 ratio in the gearbox (which is 3rd on an automatic, and 4th gear on the Tremec 6-speed on the 05 SSR)

2. The rear end ratio is changed to a umerically higher ratio, and increases if it is changed to a numerically lower ratio.

The part I am missing is the MAGNITUDE of the difference. Hence, the above request.

IF I had been smart enough to have thought of this months ago, I could have had my own SSR dynoed before and after the gear swap at Walden (They have a dyno, and used it on my SSR AFTER the swap), but I blew that chance because I just didn't think of it then.

So, if anyone else is in the process, or planning to do the swap soon, please leep this request in mind if possible.

This effect does NOT manifest itself on steady state dynos (like the Mustang dyno, for example), only on inertia type dynos.

The data would be very useful.

Interesting tech article will follow as soon as I have all my ducks in a row. You will see and understand the significance.

It is simple physics as to why the power output decreases on an inertia-type dyno. It is not due to increased friction as many folks have touted on a few Vette forums.

Running on the dyno in either a lower tranny ratio (lower than a 1:1 like you describe) or a higher numerical gearset basically means that the rear tires are spinning slower for a given engine rpm than before.

Since the inertia-type dynos measure power by calculating how fast the drum (of a known weight) is spinning and how quickly the rotational speed changes, for the same engine rpm less power is calculated. That is why there is a difference and why it is lower.

In other words, if with the stock gearset (or in the proper gear) the rear tires will spin at say 80 mph at 2,000 rpm. Spinning the drum at 80 mph equates to say 250 HP. With the lower gearset (or in a lower gear) at 2,000 rpm, the rear tires (and drum) now only spin say 75 mph. Spinning the drum at 75 mph equates to say 243 HP (since HP is based upon how fast a known weight - the drum - can be moved a certain distance). That's why the dyno readings show lower than before.

You can observe the same thing if you change the rear tire diameter, for the same reasons.

It is not the rpm of the roller that gives the dyno computer the raw data for calculating the horsepower, but rather the RATE OF CHANGE of the rpm of the roller.

To keep the roller turning at any specific rpm onan INERTIA type dyno takes almost no power - just enough to overcome the friction within both the dyno roller system and within the vehicle's drivetrain. That is nowhere NEAR the 240 to 325 hp that the 2 versions of the SSR can generate.

What the whole idea is on an inertia dyno is to measure the RATE at which the roller is ACCELERATED. Because the computer on the dyno knows the moment of inertia of the roller, and it also knows (thanks to sensors on the roller) how quickly the roller is being acclerated by the vehicle, it can calculate the instantaneius torque being applied to the roller. Knowing the instantaneous torque, and knowing the instantaneous rpm, it can then calculate the instantaneous POWER being applied. By doing this in small increments over the entire acceleration run, it can build a power curve. It then plots this curve versus the ENGINE rpm, not the dyno rpm, so that you know the power available at each engine rpm.

Running the dyno test in a lower gear, whether via using a lower gear than the 1.0 to 1.0 ratio gear in the tranny, or because the rear axle ratio has been changed, does NOT change the power reading just because of the gear ratio change. If you follow the mathematics out in detail, you will see why.

It is rather the fact that the vehicle drivetrain is being ACCELERATED from starting rpm to peak rpm in a shorter time period, when you have numerically higher gearing (i.e. "lower gearing"), that causes the apparent "reduction" in power. The drivetrain has an overall moment of inertia, whose magnitude depends on the weight and radius of the various driveline components that are being accelerated. Those components include the wheel & tire assemblies, the rear axle rotating components, the driveshaft, all the rotating components within the transmission, the flexplate or flywheel. the complex total assembly of crankshaft with rods and pistons hung on it, and all the rotating accessories attached to the engine.

When that engine works to accelerate the roller, it also has to work to rotationally accelerate all these rotating components. With shorter (numerically higher) gearing, it has more leverage against the roller and accelerates it more quickly (check with any experienced dynovoperator - he will tell you that a vehicle with shortened gearing completes its run more quickly).

It takes WORK to accelerate those rotating components. It takes MORE work in LESS time to accelerate them on a vehicle with shorter gearing. More work in less time = more horsepower. So, more of the available engine horsepower is being diverted to spinning up these rotating components. THAT's why the reported net horsepower that is accelerating the roller is lower with shorter gearing.

That does NOT mean EITHER that (a) the engine is producing less power than before the gear change, or (b) that the fricitonal losses are higher (although they will in general be a BIT higher with shorter gearing - but not anywhere near the magnitude of change seen on the dyno). It just means that more of the available engine power is being diverted towards spinning up the rotating components instead of towards spinning up the ROLLER.

This IS manifested on the street as well. With stock gearing, and my set shoift points, my SSR accelerates the rotating drivetrain mass through ALL of 1st gear and again through ALL of 2nd gear to get to 89 mph. With 4.56 gearing, it accelerates that drivetrain mass through ALL of 1st, again through ALL of 2nd, and then through about 800 rpm of 3rd gear as well to get to that same road speed. Furthermore, it accelerates that mass more quickly than with stock gearing, because it is a full SECOND faster from 0 to 60 mph for example. It IS diverting more of its power to spinning up the rotating mass. However, that unrecoverable "diversion" is MUCH smaller than the GAINS made by (a) providing more leverage in the critical 0 to 60 mph speed range and (b) spending proportionately more of the total acceleration time in rpm ranges where there is higher horsepower being produced.

I am looking for that difference in dyno results in order to determine the amount of power being DIVERTED, and therefore then being able to estimate the moment of inertia of the entire drivetrain. (GM doesn't think we need to know that, so doesn't provide it ).

OK, I'm embarassed now. I was very tired when I posted and tried to simplify things too much.

What I was trying to state was what you have posted. The change in drum rpm gives you, through a series of calculations, a torque value. Then that is used to calculate HP. How fast the drum is spinning is correlated to engine rpm on some dynos; some plug into the PCM and will read rpm from the car.

The base rpm of the drum and the change in drum rpm and the time it takes for rpm to change are the inputs into the equations. The lower gears make the same rpms changes on the drum, but at a lower intial and ending drum rpm than before. The lower rpm values used in the equations to calculate HP, even if the torque value is the same, gives a lower HP value.

Now, part of this apparant decrease in HP is partially offset by the lower gears due to the additional torque multiplication they provide; they can change the drum rpm a little quicker than the higher gears can.

In the Vettes, the before and after dyno readings from gear swaps do not amount to a large HP difference in most cases. There, going from 3.42's to 3.73's the difference is around 10 rwhp. Jumping from 3.42's to 4.56's is more than 10 rwhp, but I can't remember the exact figure. I'll try to recover that info and post it here.

I don't exactly understand what values you are looking for. I run a DynoJet 248 at work and know quite well what happens with different gear ratios. The effeciency of different gear ratios change based on the way the gear is cut and on the actual ratio. Obiviously a 6.50 gear will absorb more HP than a 3.82 but the mechanical advantage of the 6.50 will enable the car to accelerate much faster. My dyno has an eddy current attached to it so I can do load testing and trak sim on race cars. Though we only use ford 9" rear ends at work, I have never seen a huge diffence between what the 248 says weather it is running on inertia or with the eddy currrent hooked up. Small differences (usually the eddy current is a bit higher) can be attributed to wheel slip (tire slip is less of a factor with the eddy current enabled because your not accelerating) and the inertial stability of the driveline components (they are spinning at constant rate with the eddy current hooked up). These differences are less than 2-5hp, well within the accuracy of any normal chassis dyno. That said, changing from a 3.73 to a 4.56 gear should result in about a 15-25 hp loss at peak power. A basic rule of thumb that I use at work is 30 hp for every one ratio. 4.56-3.73=.83 .83*30=25hp Also, another consideration is the fact that the drag of the gear increses with RPM so the faster you spin it the more loss you get. At work our cars reach 9700 RPM and if you overlay power plots of different gears you will see a bigger difference at the highest RPM (it is not a linear change)

I expected that the differences would be largest at high rom, as the power diverted to spin up rotating driveline components increases with rpm.

One simply dyno procedure that I THINK might show very clearly what I am looking for is to run a vehicle up through multiple gears, 1st through 3rd, rather than just running the vehicle up through ONE gear, and show all 3 power curves on one power versus time graph. I suspect that in such a 1st through 3rd run, the power versus time curves for the 3 gears would have the following properties:

the 1st gear curve would be the lowest in peak magnitude
the 2nd gear curve would be higher
the 3rd gear cruve would be the highest

And, of course, the 1st gear power versus time curve would be the shortest in terms of TIME, and the 3rd gear power versus time curve would be the longest in terms of time.

I THINK I could derive what I am looking for from such a composite run graph.

I am seeing PART of what I need on my G-Tech runs, but the G-Tech curves are polluted by the air and road drag which have to be estimated and added to the G-Tech curves to get net power curves.

Some of what you say is correct, the Power versus time plot would be shortest in 1st gear, and longest in 3rd gear but the magnitudes won't change. The gears in your transmission are almost a straight cut gears so frictional losses in the transmission gearing are nearly irrelevant (first is about the same as 3rd or 4th). Because the dyno jet software reads the drum speed and knows your RPM it calculates its own gear ratio (that takes into account Transmission gear, rear gear, and tire size). The final power curves are based upon the derived gear ratio that it calculates so wheather your in 1st,2nd, 3rd or 4th it really doesn't matter (to a point).

for you guys that know about these things to discuss them. I don't have a clue what you're talking about, but I like to read your comments. Keep up the good work, we all like it!
P.S. What brand gear set would you ALL recommend in the 4.56 ? I think I would like to do that gear change next, but all the discussion in an earlier thread left me confused.

Jim, I wish I could try it but I doubt it will ever happen because I don't really have time to do a gear swap on my SSR I do know that day in and day out the race cars I dyno have different rear gears as well as different transmission ratios and when dynoed in 4th gear each combination has a different final drive yet the power plots that WinPep7 spits out overlay within 1% or else I have problems If you ever figure it out let me know! My email is Doug@LanterPerformance.com and my website is http://www.LanterPerformance.com

1hot12c: It wasn't clear who you were asking, but if asking me, I normally try to get the dyno operator to cool the engine between runs, but with the SSR, that is sort of futile because it takes too long to dissipate the heat out of that engine compartment.

On my last dyno session, which Reese at MTI Racing did right after his team completed the camshaft swap, he had multiple concerns that made for less than ideal results:

- Brand new camshaft, so it is not a good idea to immediately run at full throttle

- Had no choice but to do it anyway after a very short break-in run

- The break-in run pre-heated the darn engine too much of course in an indoor setting on the dyno

- Reese only had a very limited amount of time to work, as his crew didn't finish the job until late Wednesday night, and I had to leave At;lanta that night and put on at least 200 miles, in order to make The SSR Homecoming in Detroit in time.

- Dynojet had diagnosed his barometer as giving erroneous readings, so he was using a handheld machine designed for dragracers, which is accurate, but required him to manually enter the barometric pressure (it has a big effect on the correction factor, and was stupidly low that night).

- It was pouring rain (high humidity which hurts results and requires correction factors for THAT by the Dynojet software)

- It was around 95 degrees as I recall, which really hurts an SSR (or any vehicle that runs hot) because the correction factors falsely assume that the engine can perform "ideally" (other than the lower air density of course, which the Dynojet correction formula handles), and our SSRs do NOT perform "ideally" at higher temperatures.

All in all, I would have to say that my last dyno session was very far from ideal in terms of conditions . . .

But, I knew as soon as I got it out on the street (pouring rain and all), it was MUCH quicker, and that dissipated some of the disgust with the dynoing conditions . . .

Since I have only done the ONE grear ratio change PERSONALLY, and I used Motive Gear parts, I can only comment on Motive Gear.

I think Motive Gear needs to reassess their recommendations on backlash, as they recommend 0.006" minimum and target 0.008", but Reese at MTI was able to get it quieter by going for 0.004" to 0.005".

Other have done the swap using Yukon and other brands. Maybe they can comment?

Jim G

My gear guy winces when he hears these numbers. He sets at twice that clearance and claims much less wear and long term problems - but I do have to put up with some whine. Really noticeable cold but goes away when warm (with expanding metal, which is why some installers like the tighter tolerances). Seems counterintuitive that you tighten the facing to reduce noise (doesn't that INCREASE friction?!?), but thats how it goes.

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